1. Field of Invention
The present invention relates to methods for fabricating transistors.
2. Description of Related Art
Recently, the use of relatively large transparent substrates composed of inexpensive glass, resins, etc., for electro-optical devices, such as liquid crystal display devices and organic EL display devices has been under study. Such substrates have relatively low heat-resisting temperatures. Therefore, techniques for fabricating semiconductor elements, such as thin-film transistors (TFTs), and various other devices integrated into such substrates by low-temperature processes have been developed. Moreover, since organic EL display devices are operated by a driving current, TFTs having higher driving performance are required, and techniques for fabricating low-temperature polysilicon TFTs are important.
A step of forming a high-quality gate insulating film of a MOS transistor is absolutely necessary to the fabrication process for a low-temperature polysilicon TFT, which is generally performed at 500xc2x0 C. or less. The quality of the gate insulating film considerably affects transistor performance. In order to form the gate-insulating film, for example, an ECR plasma enhanced chemical vapor deposition process (ECR-PECVD process) or a diode parallel plate RF plasma enhanced CVD process is used.
In an element having a layered structure, the performance of the element may be impaired by structural defects in the layers or at the interfaces between the layers. For example, in a TFT, structural defects in the gate insulating film and at the interface between the gate insulating film and the semiconductor film (MOS interface) affect electrical characteristics of the transistor and cause a variation in electrical characteristics.
With respect to a gate insulating film formed by the ECR-PECVD process so that the density of interface trap state is decreased, the flat band voltage shift is large, and the dielectric breakdown voltage is small. Therefore, a further improvement in the bulk characteristics of the gate insulating film is desired. On the other hand, with respect to a gate insulating film formed by the diode parallel plate RF plasma enhanced CVD process, although the bulk characteristics are generally satisfactory, the MOS interface includes many defects due to dangling bonds, etc. In either process, it is difficult to obtain a gate insulating film in which both the bulk characteristics and the MOS interface characteristics are satisfactory.
Accordingly, it is an object of the present invention to provide a method for fabricating a transistor having satisfactory electrical characteristics.
It is another object of the present invention to provide a method for fabricating a TFT having a gate insulating film with satisfactory electrical characteristics.
It is another object of the present invention to provide a method for fabricating a transistor in which the bulk characteristics of the gate insulating film and the interface characteristics can be simultaneously improved by a low-temperature process.
In order to achieve the objects described above, in one aspect of the present invention, a method for fabricating a transistor can include the steps of forming a semiconductor film on a substrate, forming a gate insulating film on the semiconductor film by depositing a silicon oxide using at least tetraethoxysilane (TEOS) and oxygen as source materials, forming a metal film on the gate insulating film, the metal film accelerating the decomposition of gases permeated into the gate insulating film, and performing low-temperature heat treatment to the gate insulating film.
In such a method, the defect density at the MOS interface can be decreased, the charge density in the gate insulating film can be decreased, and the dielectric breakdown voltage can be increased. Thereby, it is possible to fabricate a transistor having a high-quality gate insulating film by a low-temperature process.
As long as the metal film lies on the gate insulating film, the heat treatment may be performed before or after patterning, or in any step in the fabrication process of the transistor or until the fabrication of a panel provided with a plurality of transistors is completed.
Preferably, the method can further include the step of patterning the metal film so that the metal film is used as a gate wiring film of the transistor. Thus, the used metal film is reused as gate lines. In this way, it is possible to omit the step of forming a gate wiring film.
Preferably, the method further includes the steps of removing the metal film and forming a wiring film on the gate insulating film, the wiring film forming a gate of the transistor. Consequently, it is possible to use an appropriate gate wiring film.
Preferably, the heat treatment to the gate insulating film is performed at 300xc2x0 C. or more and in a temperature range which does not affect the semiconductor film and the substrate. Consequently, it is possible to fabricate the transistor by a low-temperature process.
Preferably, the step of forming the gate insulating film is performed so that the temperature of the substrate does not exceed 400xc2x0 C. Consequently, softening and deterioration of the substrate are prevented.
Preferably, in the heat treatment step, the metal film accelerates the decomposition or removal of impurities, such as oxygen (O2), water vapor (H2O), and hydrogen (H2), contained in the gate insulating film. Although any material which decomposes or removes impurities may be used, a metal is preferred. The metal film may be composed of, for example, aluminum, magnesium, an aluminum-magnesium alloy, or an alloy containing aluminum or magnesium. Consequently, the defect density in the silicon oxide film or at the interface between the silicon oxide film and the semiconductor film is decreased. One of the reasons for the decrease in the defect density is considered to be that dangling bonds lying in the film or at the interface are terminated by various chemical species, such as hydrogen radicals, hydroxy radicals, hydrogen ions, and hydroxy ions, generated by the catalytic action of the metal.
Preferably, the thin-film transistor is fabricated by a low-temperature process. Usually, it is difficult to form a gate insulating film having both excellent bulk characteristics and excellent MOS interface characteristics by a low-temperature process. However, in the gate insulating film of the thin-film transistor described above, both the bulk characteristics and the MOS interface characteristics are satisfactory. Consequently, it is possible to form thin-film transistors having satisfactory characteristics on an inexpensive substrate, such as a glass substrate of which area can be increased.
Preferably, the fabrication method can further include, after the step of forming the semiconductor film, the step of crystallizing the semiconductor film by laser annealing. Herein, the xe2x80x9ccrystallizationxe2x80x9d may be any one of polycrystallization, microcrystallization, and monocrystallization.
A transistor of the present invention can be fabricated by the fabrication method described above.
In another aspect of the present invention, a transistor includes a semiconductor film disposed on a substrate, a gate insulating film disposed on the semiconductor film, and a gate electrode disposed on the gate insulating film. The gate insulating film is composed of an insulator which is coated with an impurity-removing film for decomposing or removing impurities and which is subjected to heat treatment.
Preferably, the impurity-removing film contains at least one of aluminum and magnesium. Preferably, the insulator is formed by depositing a silicon-containing compound (e.g., TEOS) by a diode parallel plate plasma enhanced CVD process.
In another aspect of the present invention, in an electro-optical device, by using the transistor described above, performance is improved.
In another aspect of the present invention, an electronic apparatus includes the transistor, and the examples of the electronic apparatus include display devices, such as liquid crystal display devices and organic EL display devices, mobile phones, digital cameras, personal computers, and information processing devices.
It is possible to improve the characteristics of such electronic apparatuses using transistors having satisfactory characteristics which have been fabricated by low-temperature processes.